Sophorolipids are glycolipids which are produced by microorganisms, primarily by yeast species and in which long-chain hydroxy fatty acids are bonded to sophorose. Since sophorolipids are amphiphilic lipids having strong surface activity and excellent biodegradability, attention has been paid in recent years to the use of sophorolipids as biosurfactants. Since sophorolipids are products of microorganisms, and nonionic components are main constituents thereof, sophorolipids are highly dermatotropic. Therefore, sophorolipids are used as penetration enhancers for cosmetic products. Furthermore, since sophorolipids have excellent biodegradability and are highly effective even when added in small amounts, the use of sophorolipids is also increasing in the field of cleaning agents such as detergents for dishwashing.
Regarding the yeast species that produces sophorolipids, Starmerella bombicola [old name: Candida bombicola], which is a non-pathogenic, basidiomycetous yeast, is well known. The sophorolipids produced by Starmerella bombicola have a lactone type or acid type structure, have a critical micelle concentration of 40 to 100 mg/L, and decrease the surface tension of water from 72.8 mN/m to 30 mN/m Non Patent Literature 1). Sophorolipids show different physicochemical properties depending on the difference in structure. It has been reported that properties such as antibacterial properties and surface activity vary between the lactone type and the acid type of sophorolipids, or between different fatty acid species that constitute the sophorolipids (Non Patent Literatures 1 and 2).
In a case where sophorolipids are used as cleaning agents or cosmetic materials, competition with the surfactants that are currently used cannot be avoided. Conventionally, since general surfactants are bulk chemical agents, those general surfactants have been produced at very low cost. Therefore, reduction of the production cost of sophorolipids is strongly desired. Furthermore, in order to extend the scope of the usability of sophorolipids, production of sophorolipids having constituent fatty acids with various chain lengths is desirable.
In regard to the production process for sophorolipids, studies and improvements have been hitherto made mainly on, for example, the yield, purification methods, and foaming property-imparting technologies (Patent Literatures 1 and 2). Furthermore, there have been reported methods for producing medium-chain sophorolipids mainly having a carbon chain length of 12, by applying genetic modification to Starmerella bombicola and thereby interrupting intracellular β-oxidation metabolism (Non Patent Literature 3, Patent Literature 3). In this genetic modification, MFE 2 (or FOX-2), which is a gene that is in charge of two reactions such as a hydroxylation reaction and a dehydrogenation reaction in β-oxidation of yeast in peroxisomes (Non Patent Literature 4), is deleted, and thereby a β-oxidation reaction is stopped.    (Patent Literature 1) JP 2003-9896 A    (Patent Literature 2) JP 2014-150774 A    (Patent Literature 3) U.S. Pat. No. 8,530,206 B    (Non Patent Literature 1) Appl Microbiol Biotech, 2007, 76(1):23-34.    (Non Patent Literature 2) J SURFACT DETERG, 2006, 9, QTR 1:57-62    (Non Patent Literature 3) FEMS Yeast Res, 2009, 9:610-617    (Non Patent Literature 4) Cell Mol Life Sci, 2003, 60 (9):1838-1851